In order to implement data transmission of large capacity in an optical transmission system, wavelength division multiplexing (WDM) has been put into practical use. The WDM may transmit data by using plural optical signals different in the wavelength. For example, the WDM may transmit data by using plural wavelength channels.
FIG. 1 illustrates one example of an optical transmission apparatus that transmits a WDM optical signal. The optical transmission apparatus is provided at each node of a WDM transmission system and operates as a WDM transmission apparatus that transmits a WDM optical signal.
As illustrated in FIG. 1, the optical transmission apparatus includes a preamplifier 101, a wavelength selective switch (WSS) 102, and a post-amplifier 103. The preamplifier 101 amplifies a WDM optical signal received through a transmission path fiber. The preamplifier 101 collectively amplifies plural optical signals multiplexed in the WDM optical signal. The wavelength selective switch 102 processes each wavelength channel of the WDM optical signal amplified by the preamplifier 101. For example, the wavelength selective switch 102 may adjust the optical power of each wavelength channel in the WDM optical signal. The wavelength selective switch 102 may split an optical signal with a desired wavelength from the WDM optical signal. Moreover, the wavelength selective switch 102 may insert an optical signal into an empty channel of the WDM optical signal. The post-amplifier 103 amplifies the WDM optical signal output from the wavelength selective switch 102. Similarly to the preamplifier 101, the post-amplifier 103 also collectively amplifies the plural optical signals multiplexed in the WDM optical signal.
The WDM transmission system may transmit the optical signals based on a desired modulation system about each wavelength channel. For example, the WDM transmission system may transmit the WDM optical signal in which wavelength channels of different modulation systems mixedly exist. For example, a WDM optical signal in which a QPSK optical signal, an 8-QAM optical signal, and a 16-QAM optical signal are multiplexed may be transmitted.
As a related art, an apparatus and a method that transmit optical signals different in the modulation system and the bit rate based on a wavelength multiplexing system have been proposed (for example, Japanese Laid-open Patent Publication No. 2016-72834). Moreover, an optical transmission apparatus and a system that suppress quality deterioration of an optical signal of each modulation system in a WDM transmission system in which the optical signals of different modulation systems mixedly exist have been proposed (for example, Japanese Laid-open Patent Publication No. 2007-235412).
The waveform of the optical signal deteriorates due to the nonlinear effect of the transmission path fiber. Here, the waveform deterioration due to the nonlinear effect depends on the input optical power to the transmission path fiber and the modulation system. Thus, it is preferable for the optical transmission apparatus to control the input optical power to the transmission path fiber according to the modulation system. For example, when the multi-level degree of the modulation system is higher, the transmission penalty attributed to the nonlinear effect of the transmission path fiber becomes larger. When the input optical power to the transmission path fiber is higher, the nonlinear effect of the transmission path fiber becomes larger. Therefore, it is preferable to set the input optical power to the transmission path fiber lower when the multi-level degree of the modulation system is higher. For example, it is preferable to set the fiber input power of the 16-QAM optical signal lower when the 16-QAM optical signal is compared with the QPSK optical signal. The multi-level degree of the modulation system represents the number of bits transmitted by one symbol.
In the configuration illustrated in FIG. 1, the power of each optical signal in the WDM optical signal is adjusted by the wavelength selective switch 102. However, depending on the number of optical signals multiplexed in the WDM optical signal and the type of the modulation system, the quality of the optical signal often deteriorates due to setting the fiber input power low on the contrary. For example, when the number of optical signals whose fiber input power is desired to be set low (for example, optical signals modulated based on a modulation system with a high multi-level degree) is small relative to the total number of optical signals multiplexed in the WDM optical signal, the optical signal-to-noise ratio (OSNR) of the optical signals decreases. In this case, although the signal waveform is improved by suppressing the nonlinear effect, possibly the transmission distance becomes short due to the decrease in the optical signal-to-noise ratio. In view of the above, it is desirable that the optical signal-to-noise ratio may be set high while the nonlinear effect of an optical fiber is suppressed in an optical transmission system that transmits a wavelength division multiplexing optical signal.